OPTICAL DEVICES UTILIZING SINGLE CRYSTAL GaP OR GaAs FILMS EPITAXIALLY GROWN ON CaF{11 {11 SUBSTRATES AND METHOD OF FABRICATING SAME

ABSTRACT

Single Crystal GaP or GaAs films are epitaxially grown on CaF2 substrates by a molecular beam method. The film itself as well as the interface between the film and substrate exhibits few defects since the lattice constants of the film and substrate are substantially identical, thus making the films particularly useful as optical waveguides with reduced light scattering centers, or as nonlinear optical devices in which phase matching is readily accomplished by controlling the thickness of the film.

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1 1 OPTICAL DEVICES UTILIZING SINGLE CRYSTAL Ga? or: CaAs FILMS EPITAXIALLY GROWN oN car, suasraK'fisANfimernon or OTHER PUBLICATIONS Brit. J. Appl. Phys, 1967, Vol. 18, Single-Crystal Films of Silicon on Insulators, p. 1,379 relied on." Journal of Vacuum Science and Technology, July SAME 1 9 .,.v ocisws "Ga is. 91?. nd Y {75 Inventor: Alfred Yi cm, New Providence, GaAsI PM Ep x Films r w y Molecular NJ. Beam Deposition, p. 545-548.

{73] Assignee: Bell Telephone Laboratories, 1 Primary Examiner chafles E van Hem 5 lncprpomtedi Murray Berkeley Assistant ExaminerMichael W. Ball l Heights, NJ. Attorney, Agent, or Firm-M. J. Urbano {22] Filed: Jan. 30, 1970 r 7 BT T I, 21 App1.No.:7,022' [5 1 A S RAC Single Crystal Gal or GaAs films are epitaxially grown on CaF substrates by a molecular beam method. The {52] US. Cl....-. 117/201, 1 17/106 A, 148/1 .5, film itself as well as the interface between the film and 148/175, 350/96 WG substrate exhibits few defects since the lattice con {51] Int. Cl 844d 1/02, B44d l/l8, C23c 13/02 stants of the film and substrate are substantially identi- {58} Field of Search 117/201, 106 A; 148/15, cal, thus making the films particularly useful as optical 148/175; 350/96 .WG, 96 waveguides with reduced light scattering centers, or as nonlinear optical devices in which phase matching is 156} 7 References Cited readily accomplished by controlling the thickness of UNITED STATES PATENTS Y the film. I 3,476,593 11/ 1969 Lehrer 117/ 106 A i 12 Claims, 3 Drawing Figures UGHT 52 W AIR g may i r 1 OPTICAL DEVICES UTlLiZlNQSlNGLE CRYSTAL Ga? on GaAs FILMS .EPITAXIALLYIGROWN on car SUBSTRATES AND METHOD or FABRICATING SAME BACKGROUND OF THE lNVENTlON This invention relatesto the epitaxial growth of GaP or GaAs single crystal thin films of controllable thickness on CaF- substrates by a molecular beam method some of those difficulties. in a paper in the Journal of Vacuum Science and Technology, Vol. 6, 545 (July/August 1969), .l. R. Arthur and 1/]. LePore report the successful growth of epitaxial films of GaAs and GaPoriGaAs and a? substrates by molecular beam deposition. in an ultrahigh vacuum. This technique allows the thickness of the film to be readily controlled but as indicated in the article, the films generally contained more defects than did the substrates.

For some device applications, notably optical waveguiding, it is desirable. in order to reduce light scattering, that the thin film in which light is being transmitted (e.g., the Gal film herein) be single crystal and substantially free of defects not only in the film itself but also at the film-substrate interface. in this respect, it is desirable that the substrate be a single crystal, that the guiding film be a single crystal with a lattice constant which substantially matches that of the substrate, but that the index of refraction of the film and substrate be different to provide the light guiding effect.

In other device applications, such as second harmonic generation orparametric oscillation which utilize the large nonlinear optical coefficient of materials such as GaP to convert pump radiation .of one frequency to generated radiation of a different frequency, it is desirable that the efficiency of the nonlinear process be increased by phase matching the pump radiation to the generated radiation. For a given frequency and mode order, the velocity of propagation of radiation is proportional tothe thickness of the'nonlinear material. Thus, pump andgenerated radiation of different frequencies and/or mode-orders can be made to have the same velocity (i.e.. be phase matched) by an appropriate choice of material thickness.

it is, therefore, an object of this invention to grow single crystal epitaxial films of (3a? or GaAs of controllable thickness.

it is a specific object of this invention to grow such films on a single crystal substrate of CaF having a substantially identical lattice constant as the film but a different index of refraction.

3 SUMMARY OF THE lNVENTlON These and other objects are accomplished in accordance with an illustrative embodiment of the inventionv chamber 11 capable of effecting rotary motion of" trollable thickness are epitaxially grown on polished or cleaved CaF surfaces (preferablythe unmade in an ultrahigh vacuum by molecular beam deposition. Since the plane of CaF is l l l) and the lattice constant is identical to that of GaP (a'=5.45 A at room temperature) and only about 3.6 percent different than that of GaAs (a=5.65 A at room temperature), few defects are produced in the film'itself or at the filmsubstrate interface, especially for GaP. This structure is useful as an optical waveguide with light being guided in the Gal or GaAs by the different indices of refrac-- tion (rr=3.0 6 for GaP, n=3.26 for'GaAs, -n=l.43 for CaF Thus, the waveguide may bean asymmetrical structure, e.g., air-GaP-CaF or symmetrical, e.g.,

CaF -GaP-CaB. In addition, as mentioned previously,

the molecular beam method allows for ready control of the thin film thickness, a useful feature in providing phase matching when the Get? or GaAs film is utilized in nonlinear optical devices.

BRIEF DESCRIPTION OF THE DRAWING A These and other objects of the invention, together with its various features and advantages, can be easily understood from the following more detailed descirip- A tion taken in conjunction with the accompanying drawing inwhich: I

FIG. 1 is a partial schematic-partial cross-sectional view of apparatus for practicing the present invention;

FlG. 2 is a schematic of an asymmetrical opticalwaveguide in accordance with one embodiment of the 'inventiQn; and ff e I FIG. 3 is a schematic of a symmetrical optical waveguide in' accordance with a second embodiment of the invention, 7 I i I DETAILED DESCRIPTION I Turning now to FIG. 1, there isshown apparatus in accordance with the invention for growing epitaxial 1 films of GaP or GaAs of controllable thickness on a CaF substrate by molecular beam deposition.

The apparatus comprises a vacuum chamber 11 having disposed t erein a gun port 12 containing a cylindrical gun 13, typically a Knudsen cell, and a substrate.

holder'17, typically a molybdenum block, connected by means of shaft 19 to a control knob 16 exterior to holder 17. Optionally, a plurality of guns may be con tained within the gun port in cases where it is desired to heat different source materials separately. Also shown disposed within chamber 11 is a cylindrical liquid nitrogen cooling shroud 22 which surrounds gun 13 I and a collimating frame 23 having a collimating aperture 24. A movable shutter 14 is disposed in front of aperture 24. Substrate holder 17 is provided with-an internal heater 25 and with clips 26 and 27 for affixing a I substrate member 28 thereto. Additionally, a thermocouple is disposed in aperture 31 in the side of sub- -strate 28 and is coupled externally via connectors 32-33 n order to sense the temperature of substrate '28. Chamber 11 also includes an outlet 34 for evacuating the chamber by means of apump 35.

- in which single crystal Gal or GaAs thin films of con- A typical cylindrical gun 13 comprises a-refrac tory crucible 41 having a thermocouple well 42 and a thermocouple 43 inserted therein for the purpose of determining the temperature of the material contained therein. Thermocouple 43 is connected to an external 3 v detector (not shown) via connectors 4445. Additionally, the crucible 41 has a source chamber 46 in which source material (e.g., bulk GaP) is inserted for evapo- Qration by heating coil 47 which surrounds'the 'crucible.

The end of crucible 41 adjacent aperture 24 is provided with a knife-edge opening 48 of diameter preferably less than the average mean free path of atoms in the source chamber.

For purposes of exposition, the present invention will be described in detail by reference to an illustrative exponents therein in directions other than those desired,

thereby permitting the desired beam to pass through the collimating aperture24 to effect reaction at the substrate surface. Growth. is continued for a time period sufficient to yield an epitaxial film of the desired thickness, afeature of the subject technique residing in ample wherein the various operatingparameters are given.

The first step in the inventive technique involves selecting a single crystal CaF member which may readily be obtained from commercial sources. One major surface'of the CaF substrate member is initially polished or cleaved by well-known methods along the (l l 1) plane. While the use of otherplanes is possible, the i l l .plane is preferred since it has approximately the same number of atoms and the same atom spacing as that of GaP and GaAs. If the substrate is polished, any

less than torr and preferably to a value of the.

strate surface. The next steps in the process involve introducing liquid nitrogen into the cooling shroud via entrance port 49 and heating the substrate member to the. growth temperature which ranges from 500C-600C dependent upon the specific material to be grown. such range being dictated by considerations relating to arrival rates and surface diffusion.

Following, the gun l3 employed in the system, which has previously been filled with'the requisite amounts of I order of 10 to l0 torr, thereby precluding theintroduction of any deleterious components onto the subthe controlled growth of films of thickness ranging from a single monolayer (about 3 A) to more than The reason which tioned temperature ranges can be understood as follows. It is now known that Group lll-V elements contained in compound semiconductors are adsorbed upon the surface of single crystal semiconductors at varying rates, the V elements typicallybeing almost entirely reflected therefrom in the absence of Ill elements. However, thegrowth of stoichiometric lII-V semiconductor compoundsmay be effected by providing vapors of Group III and V elements atthe substrate surface, an excess of Group V element being present with respect to the ill element, thereby assuring that the entirety of the III element will be consumed while the nonreacted V excess is reflected. in this connection, the aforementioned substrate temperature range is related to the arrival-rate and surface mobility of atoms striking the surface, i.e., the surface temperature must be high enough (500C,600C) that impinging atoms have enough thermal energy to be able 10 migrate to favorable surface sites (potential-wells) to form the epitaxial layer. The higher the arrival rate of theseimpinging atoms, the higher must be thesubstrate temperature. For substrate temperatures less than 500C twinning at the surface tends to occur, whereas for temp'eratures greater than 600 C facets of (l 10) faces tend to occur in the surface. in addition, noncongruent evaporation of GaP occurs at temperatures greater the constituent of the desired films to be grown, is Y heated to a temperature rangingfrom 730C-l,000C sufficient to vaporize the .contents thereof to yield (with-shutter 1-4 open) .a molecular beam; thatis, a stream of atoms manifesting velocity components in the same direction, in thiscase toward the substrate surface. The atoms of molecules reflectedfrom the surface strike the interior surface of the cooled shroud 22 and are condensed. thereby insuring that only atoms or molecules from the molecular beam impinge upon the surface.

For the purposes of the present invention, the amount of source materials (On? or GaAs) furnished to the gun 13 must be SllffiClCflllO provide an excess of P, or A5 with respect to Ga. This condition arises from efficient of the several materials; namely, unity for Ga and l 0 for P, on Ca? surface, the latter increasing to Example l than 680C. Similarly, the cell temperature must behigh enough 730C) to produce appreciable evapo- I ration and yet not sohigh l,OOOC) that the higher arrival rate of the V element will result in most of the -V element being reflected from the surface before being trapped there by the ill element.

The followingexamples of the present invention are) givenby way of illustration and are not tobe construed;

as limitations, many variations being spirit and scope of the invention.

possible within the This example describes a process'for the growth of an epitaxial film of single crystal gallium phosphide upon a calcium fluoride substrate member.

A calcium fluoride substrate member obtained from commercial sources was cleaved along its l l l plane by conventional techniques and inserted in an apparatus of the type shown in FIG, 1. ln the apparatus actually employed. a single graphite Knudsen cell was contained in the gun port, one gram of gallium phosphide polycrystals being placedin the source chamber of the .the large differences in sticking (i.e., condensation) counity when there is an excess of Ga on the surface.

Therefore, as long as the P arrival rate is higher than that of Ga, the growth will be stoichiometric. Similar considerations apply to Ga and As.

Growth of the desired epitaxial film is effected by directing the molecularbeam generated by gun 13 at the collimator 23 which functions to remove velocity comcell. Following, the vacuum chamber was evacuated to a background pressure of the order of 10 torr and the,

substrate, with its cleaved (111) surface (about 1 cm,

X 1 cm) facing the gun, was preheated to a temperature of approximately 537C (810K) for about 10 minutes prior .to'deposition. Low energy electron diffraction showed that this thermal treatment resulted in hexagonal diffraction which is expected for a clean CaF su r face. Accurate measurement of the substrate temperature, which is important to molecular beam epitaxial dictates the, use of the aforernem I growth, was accomplishedby imbedding a chromel .alumel thermocouple in a hole mil in diameter in the substrate. Atungsten-S percent versus tungsten-26 percent rheniumthermocouplewas used for measurement of the Knudsen cell temperature. The thermocouple reading for the cell was calibrated with a pyrometer looking directly into the-effusion orifice. At this time, liquid nitrogen was introduced into the cooling shroud and the Knudsen cell heated to a temperature of 903C I (l,l76l(), thereby resulting in vaporization of the gallium phosphide polycrystals contained therein and the consequent flow of molecular beams toward the collimating frame which removed-velocity components in the beams which were undesirable. With the shutter open, the beams were focused upon the substrate surface for a period of 40 minutes, so resulting in the growth of an epitaxial film 200 A in thickness of gallium phosphide upon the substrate. With the shutter open for 4 minutes and 400 minutes, films of A and 2,000 A thickness, respectively, were readily obtained.

The lateral dimensions of the film may be controlled by well-known masking techniques or'merely by appropriate choice of the substrate size.

At the above temperatures the molecular beam consisted of three species: Ga, P and P with the corresponding vapor pressures in the cell being 5.4 X 10' atm, 5.4 X 10' atm and 5.4 XlO atm. The arrival rates of these three species on the Calsubstrate located l0 cm. away from the Knudsen cell can be readily'calculated to be 2.65'X l0 Ga/cm lsec, 2.82 X IO P lcm /sec and 1.99 X l0 P /cm /sec. Notwithstanding these differences in arrival rates, the GaP films were shown to be stoichiometric (i.e., single crystal) and to exhibit the properties of bulk GaP on the basis of both electron diffraction measurements of lattice spacings used to determine the single crystal structure and optical measurements of transmittance which provide information concerning the electronic band structures of the film- I Example II The procedure of Example I was repeated at the same temperatures but with the source chamber containing one gram of GaAs polycrystals also obtained from commercial sources. With the cleaved CaF substrate once again positioned 10 cm. away from the Knudsen cell, the molecular beam consisted of three species: Ga, A52, and As having arrival rates of about 3.93 X [0' Ga/cm /sec, 5.03 X l0 As /cm' /sec and l.l9 X l0 AsJcm /sec. A film "of 200 A thickness was produced with theshutter open for 26 minutes. Using The above examples showed that the minimum substrate temperature for the growth of untwinned epitaxial single crystal films of Ga were about 65lower for GaP grown on a GaP film than for GaP on a cleaved CaF surface. Similarly, the minimum temperature was about 105 lower when a polished CaF surface was used. Therefore, to grow GaP on CaF at reduced temperatures, the first monolayers (approximately/ Y should be grown at an elevated substrate temperature (e.g., 590C for polished CaF and 550C for cleaved CaFg) and then the substrate temperature could be'decreased by 65 or 105 depending on whether a cleaved or polished substrate is used. Optical Waveguides f It is to be understood that the above-described arrangements are merely illustrative of the many possible specific embodiments which can be devised to represent application of the principles of the invention. Nu-

merous and varied other arrangements can be devised r in accordance with these principles by those skilled in the'art without departing from the spirit and scope of the invention. In particular, the lack of defects -(i.e., light scattering centers) in the single crystal films grown as described above, in combination with the near'identity of lattice constants between the films and the Calsubstrate, make these films especially attract ive for use as optical waveguides. An asymmetrical waveguideis shown in FIG. 2 as comprising a CaF subsuite '50 -iipon which is grown as previously describedan epitaxial thin film 51 of single crystal GaP. Two parallel interfaces, 52 and53, are formed by differing indices of refraction. The former is an air (or other gas, or vacuum) GaP interface while the latter is a GaP-CaF interface. Light is directed into one end of the Gal film which guides thelineifi plaiie parallel' to the interfaces. Similarly, FIG. 3 shows a symmetrical waveguide formed by a substrate on which is grown as previously described an epitaxial thin film 62 of singleforeeffectively guided in a plane parallel to the interthe same techniques as in Example I, the GaAs films were also shown to be single crystal and therefore stoi-j chio'metric.

Example Ill e Each of Examples l and II were repeated using, however, a CaF- crystal'polished along its l l 1) plane with v diamond paste. The polished crystal was then repeatedly rinsed in acetone and finally in ethyl alcohol before mounting in the apparatus of FIG. 1. The CaF surface thus prepared exhibited polycrystalline diffraction patterns which, however, changed to asingle crystal pattern. after thermal cleaning and annealing at 650C for 5 minutes. Molecular beam deposition ofGaP and Gre ts on this polished surface produced single crystal diffraction pattems similar to those cleaved CaF- surface was used. Example IV produced when a faces.

The above-described GaP waveguides are useful in guiding light of radiation ranging from about 0.55 ,u in

the green-visible to 2-5;. in the infrared. Similar structures utilizing GaAs are capable of guiding'light in the infrared ranging from about 0.9;1. to 2-5 1. 55 v What is claimed is: 1. A method for the growth of an epitaxial single crystal film of a material selected from'the group consisting of gallium-arsenide and gallium phosphide upon a'single crystal calcium fluoride surface which,comprises the steps of reducing the background pressure to a subatmospheric pressure, preheating'said substrate to a temperature'within the range of 500C600C, and focusing at least one collimated molecular beam comprising the constituent components of the desired epi taxial film upon said preheated substrate for a time period sufficient to effect growth of afilm of said material of the desired thickness.

2. The method of claim 1 wherein said calcium fluoride substrate possesses an atomically clean surface upon which said film is grown. v I

3. The method of claim 2 wherein said background pressure is less than 1 X torr.

4. The method of claim 2 wherein said calcium fluo-' ride surface is a cleaved (l l 1) surface.

5. The method of claim 4 wherein said material is I Ga? and including the steps of initially heating said,

substrate to a temperature within said range of SCOT-600C until approximately 50 monolayers of epgun'member containing the constituent components of the desired epitaxial film to a temperature sufficient to v vaporize said components and permitting the resultant itaxialGaP are formed on said surface and then reducing said temperature by about 65.

6. The method of claim 2 wherein said calcium fluoride surface is a polished l l 1) surface.

7. The method of claim 6 including the step ofpreheating said substrate to a temperature greater than is heated 600C for a period of time sufficient to clean and anneal .said surface. 8. The method of claim 7 wherein said material is Gal and including the subsequent steps of heating said substrate to a temperature within said range of 500C600C until approximately'SO monolayers of epitaxial 0a? are formed on said surface and then reducing said temperatureby about 105. t,

9. The method in accordance with claim 1 wherein said molecular beam is formed by heating at least one vapor to impinge upon a collimating frame.

10. The method of claim 9 wherein said gun member to a temperature in the range of 730C-1,000C. I I

11. The. method in accordance with claim 9 wherein said gun member contains gallium arsenide.

I 12. The method in accordance with claim 9 wherein said gun member contains gallium phosphide. 

2. The method of claim 1 wherein said calcium fluoride substrate possesses an atomically clean surface upon which said film is grown.
 3. The method of claim 2 wherein said background pressure is less than 1 X 10 9 torr.
 4. The method of claim 2 wherein said calcium fluoride surface is a cleaved (111) surface.
 5. The method of claim 4 wherein said material is GaP and including the steps of initially heating said substrate to a temperature within said range of 500*C-600*C until approximately 50 monolayers of epitaxial GaP are formed on said surface and then reducing said temperature by about 65*.
 6. The method of claim 2 wherein said calcium fluoride surface is a polished (111) surface.
 7. The method of claim 6 including the step of preheating said substrate to a temperature greater than 600*C for a period of time sufficient to clean and anneal said surface.
 8. The method of claim 7 wherein said material is GaP and including the subsequent steps of heating said substrate to a temperature within said range of 500*C-600*C until approximately 50 monolayers of epitaxial GaP are formed on said surface and then reducing said temperature by about 105*.
 9. The method in accordance with claim 1 wherein said molecular beam is formed by heating at least one gun member containing the constituent components of the desired epitaxial film to a temperature sufficient to vaporize said components and permitting the resultant vapor to impinge upon a collimating frame.
 10. The method of claim 9 wherein said gun member is heated to a temperature in the range of 730*C-1,000*C.
 11. The method in accordance with claim 9 wherein said gun member contains gallium arsenide.
 12. The method in accordance with claim 9 wherein said gun member contains gallium phosphide. 